Non-destructive examination of paint layers enables the inspection and assessment of paint flaws, including bulges and detachment, without compromising the structural integrity of the item being analyzed. This article presents a comprehensive overview of the significance of evaluating and analyzing paint layers as well as the noninvasive testing methods employed in the process.
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Why Test and Analyze Paint Layers?
Several external factors, such as exposure to light, temperature changes, inappropriate handling, insect infestation, and bacterial contamination, can contribute to a change in the painting’s physical condition. Moreover, with aging, the paint layers become more brittle, leading to paint cracks or detachment of its support.
Untreated, a painting may deteriorate to the point of being unrecognizable or even irreparably damaged. To effectively preserve or restore paintings, it is crucial to have a deep understanding of the physical and chemical processes that lead to degradation.
The impacts typically observed during the degradation of a painting include the following:
- Color Change and Darkening: Accumulation of common room dust, organic particles, and soil particles on the surface can cause color change. Additionally, other physical processes, such as varnish oxidation or chemical reactions within paint layers, may also result in color alteration and darkening.
- Craquelure: Crackles or fine crack networks that appear on a painting’s surface are primarily caused by the shrinkage of the paint film or varnish, although it could also be attributed to the weakening of the ground or support. These cracks may have a detrimental impact on the aesthetic appeal of paintings.
- Layer separation: The separation of layers in paintings can occur between the layers of paint or paint support. This can be attributed to the fact that different types of paints have varying expansion properties, and the ground and paint layers have different stiffness values. The primary reason for this is aging, which causes alterations in the stiffness of the paint layers and a loss of stiffness of the ground. Consequently, stress builds up and causes delamination.
- Biodeterioration: Some paintings feature biological elements that can serve as nutrition sources for various creatures, such as insects, larvae, or even smaller mammals. For instance, woodworms or larvae of specific wood-boring beetles, such as Ambrosia beetles (Scolytidae, Platypodidae), house longhorn beetles (Hylotrupes bajulus), death watch beetles (Xestobium rufovillosum) and others, often bore into wood stretches, wooden frames or supports, causing damage and potentially leading to severe layer separation.
- Biodegradation: Bacteria and fungi affect the paintings significantly by biodegradation. These organisms can harm paintings by growing into them and their grounding, as well as through their metabolic processes. This can lead to the weakening of the ground, separation of layers, or deterioration of wooden support or canvas.
Non-destructive Testing Methods For Paint Layer Analysis
There are various non-destructive methods used in analyzing paint layers; a few are mentioned below:
- Optical Coherence Tomography (OCT): OCT is a non-invasive imaging technique that employs light to detect the intensity and delay of backscattered light and is based on low-coherence interferometry. This technique is similar to ultrasound imaging technology and is used on paintings to examine the cross-section of paint and varnish layers without the need for contact or sampling.
OCT is a three-dimensional (3D)-scanning Michelson interferometer specifically designed for the examination of the eye and other biological tissues. It uses the low-coherence interferometry principle, which requires a short-coherence length or wide-band sources to achieve a high-depth resolution.
OCT can double the penetration depth compared with confocal microscopy in highly scattering samples, such as paint layers, because of its ability to take advantage of the coherence properties of light and register only coherent signals.
OCT systems have a comfortable remote working distance. The optical configuration of the OCT system uses two single-mode directional couplers with a superluminescent diode as the wideband source.
- Photoacoustic Microscopy: Photoacoustic microscopy is a widely used imaging technique that utilizes light-induced ultrasonic acoustic waves. The light is generated from a pulsed source interacting with a medium to create images of the spatial distribution of the absorbing components.
This method offers better transmission than visible and near-infrared light through strongly scattering media, resulting in higher sensitivity and optical absorption contrast at a high spatial resolution.
Researchers have used this technology to create photoacoustic images of miniature oil paintings, revealing hidden pencil sketches beneath the layers of paint. Photoacoustic imaging can be adapted to study a variety of objects with varied optical properties and geometrical profiles, such as manuscripts, glass objects, plastic art, and stone sculptures, by adjusting the detection bandwidth of optically induced ultrasonic waves.
- Terahertz Imaging: This method of evaluation does not have an impact on the paint layers and relies on terahertz pulses to bounce off the layers. Because the paint layers were very thin, the pulses frequently converged with each other. This analysis is beneficial because it reveals the initial indications of corrosion beneath the paint layers before any noticeable blisters appear on the surface.
Terahertz imaging is achieved using terahertz radiation to measure the changes in roughness caused by corrosion. The height of a blister can be calculated by measuring the time difference between the reflections of a pulse from an area without a blister and from an area with one blister.
- Infrared Reflectography: This nondestructive method leverages infrared radiation to visualize and reveal hidden elements beneath the surface of paintings. By penetrating the paint layers, compositional changes, preliminary sketches, and the artist's modifications made during the creative process.
Infrared reflectography is an indispensable tool for understanding an artist's techniques and creative intentions, shedding light on the evolution of a masterpiece. Conservators can use this information to make informed decisions during the restoration process, thereby preserving the artist's original vision while ensuring the enduring nature of the artwork for future generations.
In conclusion, non-destructive analysis of paint layers plays a crucial role in understanding and addressing various defects that paintings may experience over time. The deterioration of paintings caused by factors such as exposure to light, temperature changes, handling, and biological contamination can result in color changes, craquelure, layer separation, biodeterioration, and biodegradation.
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References and Further Reading
Maev, R. G., Gavrilov, D., Maeva, A., & Vodyanoy, I. (2008). Modern non-destructive physical methods for painting testing and evaluation. In Proceedings of the 9th International Conference on NDT of Art. https://www.ndt.net/article/art2008/papers/042Maev.pdf
Zhang, N et al. (2016). Characterization of automotive paint by optical coherence tomography. Forensic Science International, 266, 239-244. https://pubmed.ncbi.nlm.nih.gov/27341546/
T serevelakis, G. J., Vrouvaki, I., Siozos, P., Melessanaki, K., Hatzigiannakis, K., Fotakis, C., Zacharakis, G. (2017). Photoacoustic imaging reveals hidden underdrawings in paintings. Scientific reports, 7(1), 747. https://www.nature.com/articles/s41598-017-00873-7